1. Field of the Invention
The present invention relates to rotary fluid pressure devices, and more particularly, to a parking lock for such devices.
2. Description of the Related Art
In many vehicle applications for low-speed, high-torque gerotor motors, it is desirable for the motor to have some sort of parking brake or parking lock, the term “lock” being preferred because it is intended that the parking lock be engaged only after the vehicle is stopped. In other words, such parking lock devices are not intended to be dynamic brakes, which would be engaged while the vehicle is moving, to bring the vehicle to a stop.
For many years, those skilled in the art have attempted to incorporate brake and lock devices into gerotor motors, as opposed to merely adding a brake package on the motor output shaft. Examples of such devices are illustrated and described in U.S. Pat. Nos. 3,616,882 and 4,981,423. In the device of U.S. Pat. No. 3,616,882, a braking element is disposed adjacent the forward end of the gerotor star, and is biased by fluid pressure into frictional engagement therewith. Such an arrangement involves a certain degree of unpredictability of performance, in view of variations in clearances, etc. Such an arrangement also requires a substantial redesign of the wear plate and forward bearing housing of the motor. In the device of U.S. Pat. No. 4,981,423, there is a multi-disc brake assembly which is of the “spring-applied, pressure-released” type. The arrangement of the '423 patent also requires almost total redesign of the forward bearing housing, and also results in a much larger bearing housing. In addition, the disc pack is in splined engagement with the output shaft and, therefore, must be able to brake or hold the full output torque of the motor, thus necessitating that the discs, the spring, and the apply/release piston all be relatively larger.
Another example of the incorporation of brake and lock devices into gerotor motors is illustrated and described in U.S. Pat. No. 6,062,835, assigned to the assignee of the present invention and incorporated herein by reference. In the device of the '835 patent, a lock piston is disposed in an internal chamber of an end cap assembly, located immediately adjacent to the gerotor gear set: A spring biases the lock piston into engagement with the gerotor gear set when hydraulic pressure is not supplied to the device. When hydraulic pressure is supplied to the device, this hydraulic pressure acts against the lock piston to disengage the piston from the gerotor gear set. Although the device in the '835 patent is compact and would function successfully in many hydraulic applications, some current manufacturers of hydraulic applications, including but not limited to mini-excavator manufacturers, have placed greater size restrictions on gerotor motors while still requiring a parking brake or parking lock feature.